1. Field of the Invention
The present invention relates to a two-component dry type developer for developing latent electrostatic images formed on a latent-electrostatic-image-bearing member in the fields of electrophotography, electrostatic recording and electrostatic printing.
2. Discussion of Background
Image formation processes employed in the field of electrophotography are widely known. Generally, a photoconductor is charged by corona charge, and exposed to light images corresponding to original images. The portions exposed to the light images become electroconductive, so that electric charges dissipate therefrom. As a result, the unexposed portions remain in the form of latent electrostatic images on the photoconductor. When a toner which is charged to the opposite polarity to that of the latent electrostatic images formed on the photoconductor are brought near the latent electrostatic images, the toner is electrostatically attracted to the latent electrostatic images, so that the latent electrostatic images are developed to visible toner images. The visible toner images are then transferred to an image-receiving sheet, and fixed thereon.
To develop latent electrostatic images, a one-component type developer comprising a toner component and a two-component type developer comprising a toner component and a carrier component are used.
When latent electrostatic images are developed by the two-component type developer, toner particles with insulating properties are triboelectrically charged to a predetermined polarity by bringing the toner particles into contact with magnetic carrier particles. At the same time, a magnetic brush is formed by the triboelectrically charged toner particles and the carrier particles. The latent electrostatic images are developed into visible toner images with the toner particles contained in the magnetic brush by bringing the magnetic brush into slide contact with the latent electrostatic images formed on the photoconductor.
With the two-component type developer, it is preferable that the materials for the carrier particles and for the toner particles be appropriately selected, with the triboelectric series thereof taken into consideration. When the material for the carrier is too separated from the material for the toner in the triboelectric series, the attraction between the toner particle and the carrier particle is so strong that the attraction between the toner particles and the carrier particles exceeds the attraction between the toner particles and the latent electrostatic images to be developed. The result is that the obtained image density is low.
The image density can be increased by increasing the toner concentration in the developer. However, when the toner concentration is excessively increased, the toner particles tend to stick together and to be deposited in non-image areas on the photoconductor.
The image density may also be increased by increasing the electric charge applied to the photoconductor to maintain the potential thereof at a high level. In this case, however, a large quantity of electric power is consumed to maintain the high potential of the photoconductor. Moreover, in the case where the potential of the photoconductor is high, even the carrier particles in the developer are deposited on the photoconductor. When the carrier particles are deposited on the surface of the photoconductor, the carrier particles tend to be transferred to a transfer sheet, so that the so-called "carry-over" of carrier takes place, and the surface of the photoconductor is scratched by the carrier particles in the course of the image transfer operation and cleaning operation.
It is most desirable that the triboelectric properties of the surface of the carrier be controlled, while maintaining the desirable physical properties of the toner and the carrier, when the developer is used in practice. One of the most significant factors which affect the stability of the triboelectric properties of the carrier is whether or not toner particles easily adhere to the carrier particles. Namely, when the developer is used in repetition, the toner particles held on the carrier particles are fused with the surface of the carrier particles or brought into pressure contact with the toner particles, by the collision between the carrier particles and various mechanical parts in a development unit. As the fused toner particles are accumulated on the surface of the carrier particles, the triboelectric chargeability of the carrier is changed, and the toner-holding capability of the carrier particle decreases so that the development performance of the developer eventually decreases.
In U.S. Pat. No. 3,942,979, there is disclosed a two-component type developer Which comprises (i) carrier particles with a specific surface area of at least about 150 cm.sup.2 /g and (ii) a toner comprising in terms of numerical percentage about 30% or less of toner particles with an average particle diameter of about 5 .mu.m or less, about 25% of toner particles with an average particle diameter of about 8 to 12 .mu.m, and 5% or less of toner particles with an average particle diameter of about 20 .mu.m or more.
Generally in cascade development, carrier particles with an average particle diameter of about 30 to 1,000 .mu.m are used, while in magnetic brush development, carrier particles with an average particle diameter of about 30 to 250 .mu.m are used.
Commercially available developers for magnetic brush development comprise carrier particles with an average particle diameter of about 100 to 200 .mu.m and toner particles with an average particle diameter of 1 to 30 .mu.m. These developers, however, do not meet the requirements that images be produced with high image quality for an extended period of time.
To improve the image quality of copied images, it is important that the specific triboelectric charge quantity of the toner and carrier be within an optimal range. As for a two-component type developer, the specific triboelectric charge quantity is usually measured by a blow-off method, which measures the quantity of electric charge generated between the toner particles and the carrier particles per unit weight of the toner particles. Hereinafter, the specific triboelectric charge is simply referred to as the specific triboelectric charge of toner particles. The higher the value, the greater the quantity of the triboelectric charge generated between the toner particles and the carrier particles. When the value of the specific triboelectric charge quantity of the carrier is high, an electric field with high intensity is required to develop latent electrostatic images formed on the photoconductor with the toner particles, because the toner particles have to be separated from the carrier particles with large force for developing latent electrostatic images. The force required for separating the toner particles from the carrier particles is determined by the intensity of the electric field between the photoconductor and a development sleeve for supporting the developer thereon, which is directed toward the photoconductor.
As previously mentioned, when the specific triboelectric charge quantity of the toner exceeds an optimal level, the toner cannot be sufficiently transported onto the photoconductor even if the intensity of the electric field between the photoconductor and the development sleeve is set at a normal value. The result is that images obtained have low image density. On the other hand, when the specific triboelectric charge quantity of the toner is lower than the optimal level, the attraction force between the carrier particle and the toner particle is so weak that the toner particles are easily separated from the carrier particles and transported onto the photoconductor, so that images with high image density can be obtained. However the toner particles are easily scattered even by an air stream caused by the rotation of the development sleeve. The result is that the scattered toner particles stain the inner parts of the development unit.
Furthermore, such toner particles are deposited not only on image areas, but also on non-image areas of the photoconductor, so that the so-called fogging occurs in the images obtained. As mentioned previously, a developer comprising toner particles and carrier particles, with a low triboelectric charge quantity, however, has an advantage that high image density can be obtained. This is because a large amount of the toner particles can be transferred to the photoconductor even when the intensity of the electric field between the photoconductor and the development sleeve is not high.
More specifically, when the triboelectric charge quantity of the toner is 10 .mu.C/g or less, the toner particles are considerably scattered in the development unit although images with high density can be obtained. In contrast to this, a developer which comprises toner particles and carrier particles with a specific triboelectric charge quantity of 25 .mu.C/g or more is not capable of producing images with high image density although the scattering of the toner particles can be avoided. With the above-mentioned advantages and disadvantages taken into consideration, most of the commercially available developers have a specific triboelectric charge quantity ranging from 10 or more to less than 25 .mu.C/g. However, it is extremely difficult to maintain the specific triboelectric charge quantity in the above-mentioned range while in use. In particular, when the developer is repeatedly used for copying operation, the toner particles are fused to the surface of the carrier particle. As a result, the triboelectric effect between the toner particle and the carrier particle declines, so that the charge quantity of the toner is decreased and the scattering of the toner particles tends to take place.